The changes in gene expression which define cellular differentiation are regulated largely at the level of transcription. Therefore, in order to unravel the molecular mechanisms controlling embryogenesis it is important to understand the impact of specific transcription factors in defining cell fate. It is difficult to address this in many organs because they consist of multiple tissues containing many cell types all of which interact to control organogenesis. However, the liver provides a relatively less complicated system because hepatocytes make up 90 percent of its mass. In addition, intense study of liver promoters has identified a number of transcription factors which may have important roles in regulating liver gene expression. The liver develops from a portion of the foregut endoderm which receives an inductive signal from the adjacent heart mesenchyme. As a result of this HNF-4 expression is initiated in endoderm cells destined to follow a hepatic fate. We have shown that mouse livers which lack HNF-4 fail to express many genes required for normal liver function and have concluded that this factor is crucial for hepatocyte differentiation in vivo. However, specification of the hepatic lineage is not affected by loss of HNF-4 suggesting that factors acting upstream of HNF-4 may play important roles during the earliest stages of liver development. The ultimate goal of this grant is to identify these liver specification factors. We have proposed that the visceral endoderm and liver utilize similar mechanisms to regulate their development. The transcription factors (GATA-4 and GATA-6 have recently been shown to act upstream of HNF-4 during visceral endoderm development. In addition, because these factors are also expressed in the developing liver makes them good candidate liver specification factors. In aims 1 and 2 we will analyze their contribution toward early liver development. In aim 3 we propose to identify additional candidate hepatic specification factors by analysis of the HNF-4 promoter. Using transgenic mice we will identify the regulatory elements necessary for expression of HNF-4 in the liver diverticulum. From this information we will ascertain which factors interact with these elements and determine their contribution to development of the liver lineage.